This application is a 371 of PCT/EP 99/07176, filed Sep. 28, 1999.
The present invention relates to a process for preparing water-swellable hydrophilic polymers, to the polymers obtained thereby and to the use of these polymers.
Hydrophilic hydrogels are obtainable by polymerization of unsaturated acids, for example acrylic acid, methacrylic acid, acrylamidopropanesulfonic acid, etc., in the presence of small amounts of multiply olefinically unsaturated compounds are already known as superabsorbent polymers.
Also known are hydrophilic hydrogels obtainable by graft copolymerization of the olefinically unsaturated acids onto different matrices, for example polysaccharides, polyalkylene oxides and derivatives thereof.
The hydrogels mentioned are notable for high absorbency for water and aqueous solutions and are therefore widely used as absorbants in hygiene articles.
Such water-swellable hydrophilic polymers are generally prepared by free-radical polymerization in an aqueous solution which contains the monomers with or without a grafting base and crosslinker.
The water-swellable hydrophilic polymers produced for use in the hygiene and sanitary sector have a degree of neutralization in the range from 60 to 85 mol % based on the polymerized acid-functional monomer units, so that the hydrogels formed in use are pH neutral with regard to the skin.
The degree of neutralization is generally set prior to the polymerization, since this avoids the technically difficult neutralization of an acidic hydrogel of high viscosity. However, the polymerization of, for example, acrylic acid in the neutral pH range is slower, and leads to lower molecular weights, than the polymerization in the acidic range. This is explained by the electrostatic repellency between the most recently incorporated monomer unit and the next monomer unit to be incorporated, which repellency arises only minimally, if at all, in the case of a polymerization in the acidic pH range, since the monomer units are present in the uncharged, acidic form.
The trend toward ever thinner diaper constructions requires water-swellable hydrophilic polymers providing better and better performance characteristics with regard to absorption capacity, gel strength, gel permeability and residual extractables.
The desired combination of high absorption, high gel strength, high gel permeability and low residual extractables can only be provided by a polymerization where very high molecular weights are obtained for the primary polymer chains. The preferred way to provide such products is therefore a polymerization in aqueous solution where the acid-functional monomer units present in the monomer solution are only partly preneutralized, if at all. The degree of neutralization of the acid-functional monomers is preferably in the range from 0 to 40 mol %, particularly preferably in the range from 0 to 25 mol %.
Polymerization and subsequent coarse comminution provides acidic hydrogel particles which have to be adjusted to the desired ultimate degree of neutralization of 60-85 mol % based on acid-functional monomer units by neutralizing these acid-functional monomer units. This neutralization is a process which is technically difficult to carry out and which has to meet particular requirements. First, the gel must not be excessively sheared during the contacting with the neutralizing agent so as to avoid increasing the extractables content, which would have an adverse effect on the properties of the end product and accordingly is undesirable. Secondly, neutralization has to be completely homogeneous in order that sufficiently good drying characteristics may be obtained for the gel particles. This is because acidic hydrogel particles having a low degree of neutralization are very tacky and are incapable in the subsequent belt drying of forming the loose assembly that is needed if high drying rates are to be obtained.
The subsequent neutralization of acidic hydrogels is known in principle.
DE-A-26 12 846 discloses a process for preparing a water-absorbing resin by polymerizing at least one starch and/or cellulose with at least one water-soluble monomer having a polymerizable double bond and with a crosslinker. The polymers obtained are neutralized with bases, although the method of neutralization is not more particularly specified.
According to EP-A-0 205 674, acidic polymers are prepared at from 0 to 100xc2x0 C., preferably from 5 to 40xc2x0 C., and their pH is adjusted by subsequent partial neutralization of the hydrogel. Neutralization is effected here by adding the gel to a very dilute sodium hydroxide solution. This method is disadvantageous, since large amounts of water have to be evaporated at the drying stage owing to the very dilute nature of the sodium hydroxide solution.
EP-A-0 303 440 describes the production of a hydrated crosslinked gel polymer which has 10 to 50 mol % of the acid-functional monomers neutralized and which is adjusted to the desired ultimate degree of neutralization by adding a neutralizing agent in a reaction vessel having a plurality of rotary shafts each fitted with stirring blades. True, this process provides homogeneous neutralization, since new surfaces are constantly being generated for the gel particles, but the shearing force on the gel is too high and leads to an undesirable increase in extractables.
EP-A-0 238 050 claims a process for the batchwise production of finely divided crosslinked water-absorbing polymers by conducting the polymerization in a kneader and having a degree of neutralization for the (meth)acrylic acid in the range from 0 to 100 mol %. The polymerization batch is neutralized to the desired ultimate pH in the kneader used for the polymerization, either during the polymerization or subsequently thereto. This again provides homogeneous neutralization, but the shearing forces applied are too high, so that an undesirable increase in the extractables content occurs.
In U.S. Pat. No. 5 453 323 and EP-A-0 530 438, acrylic acid is used together with water-soluble hydroxyl-containing polymers to prepare under adiabatic conditions and without neutralization of the monomers polymer gels which are subsequently comminuted in an unspecified meat grinder. The neutralizing agent is added to this comminuted gel and the mixture is again chopped. The postcrosslinker is then added and the gel is again chopped three times in order that all the reactants may be incorporated in the gel in a homogeneous manner. This repeated chopping of the gel exerts an undesirable shearing stress on the gel, elevating the level of extractables.
EP-A-0 629 411 describes the polymerization of acrylic acid with crosslinkers. The gel obtained is subsequently partially neutralized with an alkali metal salt and further crosslinked by addition of a crosslinker. The method of neutralization is not further specified in the reference; one example mentions kneading the gel with the neutralizing agent in an extruder.
DE-A-195 29 348 describes preparing superabsorbent polymers by polymerizing a partially preneutralized monomer solution under adiabatic conditions. The degree of preneutralization of the acid-functional monomers is in the range from 5 to 30 mol %. The acidic gel is neutralized after its comminution in simple mixing assemblies such as a rotating drum or in a Drais mixer, the aqueous solution of the bases being introduced via nozzles or spray injectors, for example. True, this avoids any mechanical damage to the polymer gel, but cannot provide homogeneous neutralization, since the gel is not destructured in the course of the mixing with the neutralizing agent. The pH inhomogeneities of the gel in turn lead to inferior drying, which is undesirable for economic reasons.
It is an object of the present invention to provide a process for postneutralizing acidic hydrogels homogeneously and with minimal shear stress on the gel to avoid an undesirable increase in the extractable fractions.
We have found that this object is achieved by the process for preparing water-swellable hydrophilic polymers by neutralization of the acidic hydrogel having a degree of neutralization of 0-40 mol % to an ultimate degree of neutralization of 60-85 mol % by mixing with a neutralizing agent in a mincer comprising a system of screw, rotating blade, restricted flow zone and breaker plate, wherein
the mincer has a power output of from 1000 to 6000 Wh/m3 
the hydrogel passes through a zone having an energy dissipation density of from 400 to 800 W/1 of mixing volume
the average residence time of the hydrogel in the mincer is from 5 to 30 seconds
the breaker plate has an open area of from 20 to 40%.
Preference is given to a process for preparing water-swellable hydrophilic polymers, which comprises
a) free-radically (co)polymerizing one or more hydrophilic monomers or graft (co)polymerizing one or more hydrophilic monomers onto a grafting base, the average degree of neutralization of the acid-functional monomers being from 0 to 40 mol %;
b) coarsely comminuting the acidic hydrogel;
c) neutralization of the acidic hydrogel to an ultimate degree of neutralization of 60-85 mol % by mixing with a neutralizing agent in a mincer comprising a system of screw, rotating blade, restricted flow zone and breaker plate, wherein
the mincer has a power output of from 1000 to 6000 Wh/m3 
the hydrogel passes through a zone having an energy dissipation density of from 400 to 800 W/l of mixing volume
the average residence time of the hydrogel in the mincer is from 5 to 30 seconds
the breaker plate has an open area of from 20 to 40%;
d) placing the neutralized hydrogel particles without further mechanical shearing stress onto a belt dryer;
e) drying the hydrogel particles using a belt dryer and
f) grinding and sieving the dried hydrogel particles.
The process of the invention will now be more particularly described.
Hydrophilic monomers useful for preparing the water-swellable hydrophilic polymers of the invention include for example acids capabale of addition polymerization, such as acrylic acid, methacrylic acid, vinylsulfonic acid, vinylphosphonic acid, maleic acid, maleic anhydride, fumaric acid, itaconic acid, 2-acrylamido-2-methylpropanesulfonic acid, 2-acrylamido-2-methylpropanephosphonic acid and their amides, hydroxyalkyl esters and amino- or ammonio-functional esters and amides. Water-soluble N-vinylamides or else diallyldimethylammonium chloride are also suitable.
Preferred hydrophilic monomers are compounds of the general formula (I) 
where
R1 is hydrogen, methyl or ethyl,
R2 is xe2x80x94COOR4, hydroxysulfonyl, phosphonyl, a (C1-C4)-alkanol-esterified phosphonyl group or a group of the formula (II) 
where
R3 is hydrogen, methyl, ethyl or carboxyl,
R4 is hydrogen, amino-(C1-C4)-alkyl or hydroxy-(C1-C4)-alkyl, and
R5 is hydroxysulfonyl, phosphonyl or carboxyl.
Examples of (C1-C4)-alkanols are methanol, ethanol, n-propanol and n-butanol.
Particularly preferred hydrophilic monomers are acrylic acid and methacrylic acid.
When the monomers used are acids, their alkali metal or ammonium salts may be used as comonomers in a fraction of up to 40% by weight.
Useful grafting bases may be of natural or synthetic origin. Examples are starch, cellulose or cellulose derivatives and also other polysaccharides and oligosaccharides, polyvinyl alcohol, polyalkylene oxides, especially polyethylene oxides and polypropylene oxides, and also hydrophilic polyesters. Useful polyalkylene oxides have for example the formula (III) 
where
R6 and R7 are independently hydrogen, alkyl, alkenyl or aryl,
X is hydrogen or methyl, and
n is an integer from 1 to 10,000.
R6 and R7 are each for example linear or branched (C1-C10)-alkyl, methyl, ethyl, propyl, isopropyl, n-butyl, (C2-C6)-alkenyl or aryl such as unsubstituted or (C1-C4)-alkyl-substituted phenyl.
R6 and R7 are each preferably hydrogen, (C1-C4)-alkyl, (C2-C6)-alkenyl or phenyl.
The hydrophilic, highly swellable hydrogels are preferably in a crdsslinked state, i.e., they containunits polymerized into the polymer network that are derived from compounds having at least two double bonds.
Usel crosslinkers include in particular methylenebisacrylamide, methylenebismethacrylamide, esters of unsaturated mono- or polycarboxylic acids with polyols, such as diacrylate or triacrylate, e.g., butanediol diacrylate, butanediol dimethacrylate, ethylene glycol diacrylate, ethylene glycol dimethacrylate and also trimethylolpropane triacrylate, allyl compounds such as allyl (meth)acrylate, triallyl cyanurate, diallyl maleate, polyallyl esters, tetraallyloxyethane, triallylamine, tetraallylethylenediamine, pentaerythritol triallyl esters or allyl esters of phosphoric acid and also vinyl compounds such as vinyl acrylate, divinyl adipate, divinylbenzene and vinylphosphonic acid derivatives, as described for example in EP-A-0 343 427.
The polymerization may be initiated using high energy electromagnetic radiation or the customary chemical polymerization initiators, for example organic peroxides such as benzoyl peroxide, tert-butyl hydroperoxide, methyl ethyl ketone peroxide, cumene hydroperoxide, azo compounds such as azodiisobutyronitrile and also inorganic peroxy compounds such as ammonium persulfate, potassium persulfate or hydrogen peroxide, with or without reducing agents such as sodium bisulfite, and iron(II) sulfate or redox systems where the reducing component is an aliphatic or aromatic sulfinic acid, such as benzenesulfinic acid or toluenesulfinic acid or derivatives thereof, for example Mannich adducts of sulfinic acid, aldehydes and amino compounds.
Polymerization in aqueous solution is preferably conducted as a gel polymerization by utilizing the Trommsdorff-Norrish effect. It is particularly preferable for the polymerization to be carried out in the quiescent state without mechanical mixing, so that the hydrogel that forms is not exposed to any mechanical shearing forces which would raise the level of extractables. The, polymerization may here be carried out not only batchwise, for example in a cylindrical reactor, but also continuously, for example by polymerization on a belt reactor.
The resultant hydrogels are coarsely comminuted by means of customary pulling and/or cutting tools, for example by the action of a discharging pump or screw in the case of a polymerization in a cylindrical reactor or by a cutting roll or cutting roll combination in the case of a belt polymerization.
The acidic hydrogel is subsequently neutralized according to the invention by destructuring and mixing the hydrogel and the neutralizing agent in a mincer comprising a system of screw, rotating blade, restricted flow zone and breaker plate and providing a power output of from 1000 to 6000 Wh/m3, preferably of from 2500 to 5000 Wh/m3, by passing the hydrogel through a zone having an energy dissipation density of from 400 to 800 W/1 of mixing volume. The process utilizes residence times of from 5 to 30 seconds. The frequency of the rotating blade is 1-5 sxe2x88x921, preferably 3-4 sxe2x88x921. To reduce the shearing forces on mixing in the restricted flow region above the breaker plate of the apparatus, the capillaries in the breaker plate are conical. The open area of the breaker plate is from 20 to 40%, preferably from 25 to 35%, and the initial hole diameter is from 4 to 16 mm, preferably from 8 to 10 mm, coupled with a conical widening at an angle of from 8xc2x0 to 20xc2x0, preferably from 10xc2x0 to 15xc2x0. A mincer is similar in equipment terms to an extruder, but exerts less shearing force.
The design described provides a combination of high mixing efficiency and of benign mechanical treatment of the mixture of hydrogel and neutralizing agent. A single-stage treatment would prove to be absolutely adequate for homogeneous distribution, avoiding the repeated mincing of the gel which would in turn lead to an undesirable increase in the shearing stress on the gel.
The choice of neutralizing agent is not critical, suitable neutralizing agents being alkali metal hydroxides, ammonia, aliphatic primary and secondary amines, alkali metal carbonates and alkali metal bicarbonates. Particular preference is given to sodium hydroxide and sodium carbonate. The neutralizing agent may be added in liquid form, for example aqueous sodium hydroxide solution, in solid form, for example sodium carbonate powder, or in gaseous form, for example ammonia.
The specific design of the mincer also makes it possible to mix other reactants or materials with the polymer gel to be neutralized according to the invention. This avoids the repeated mincing of the gel which would in turn lead to an undesirable increase in the shearing stress on the gel.
For instance, the gel may be admixed with reactants capable of reacting with free acrylic acid, for example amino acids such as cysteine or lysine, hydroxylamine and/or its salts such as hydrochloride or sulfate, hydrazine and/or its salts, ozone or sulfur compounds having a reducing effect, such as alkali metal sulfites, bisulfites or disulfiteas, sodium thiosulfate or mercapto compounds.
The gel may also be admixed with materials capable of reacting with the carboxyl groups of the hydrogel by crosslinking. Examples of such materials are polyhydric alcohols, polyacid amines, polyamidoamines and their reaction products with epichlorohydrin, di- and polyepoxides, bis- and polyaziridines, bis- and polyoxazolines, di- and polyisocyanates, ethylene carbonate or oxazolidone.
It is further possible in this stage to mix the gel with fines of superabsorbent polymers that are obtained, for example, from the production of water-swellable hydrophilic hydrogels during the grinding and subsequent classification of the dried hydrogels.
Various ways are known for drying hydrogel particles. For instance, they may be dried by the thin film drying process, for example by means of a biaxial can dryer; by the plate drying process, whereby the hydrogel polymer particles are loaded onto plates in several layers in a drying chamber in which hot air circulates; by the rotating drum process using can dryers; or by the conveyor belt process, hereinbelow also referred to as simply belt drying. Belt drying, where foraminous trays of a circle conveyor are loaded in a tunnel with the material to be dried and the material is dried by blowing hot air through the tray holes during the passage through the tunnel, constitutes the most economical drying process for water-swellable hydrophilic hydrogels and is therefore preferred. The rate of drying of the material to be dried is determined by the evaporation rate, which indicates how many kg of water evaporate per square meter of belt area per hour from the product to be dried. This evaporation rate should be as high as possible for economic reasons.
The hydrogels which have been neutralized according to the invention and which have preferably been mixed with additional reactants and/or superabsorbent fines have an economically advantagoeus drying rate for belt drying. They possess a standardized evaporation rate of at least 50 kg/m2h, preferably at least 70 kg/m2h, particularly preferably at least 80 kg/m2h in hot air drying at 180xc2x0 C. at an air velocity of 2 m/s.
In a particularly preferred process, the standardized evaporation rate can be further enhanced by applying a release agent to the hydrogel particles beforehand. The release agents are applied without mechanical stress on the hydrogel particles by spraying in suitable equipment, for example rotary tube, Drais mixer, plowshare mixers such as Lxc3x6dige mixers, Peterson-Kelly mixers, cone screw mixers, etc.
Useful release agents include nonionic, anionic, cationic or amphoteric surfactants having an HLB value of not less than 3 (for a definition of the HLB value see W. C. Griffin, J. Soc. Cosmetic Chem. 5 (1954) 249). Preference is given to surfactants which are soluble or at least dispersible in water.
Useful nonionic surfactants include for example the addition products of ethylene oxide, propylene oxide or mixtures of ethylene oxide and propylene oxide with alkylphenols, aliphatic alcohols, carboxylic acids and amines. For example C8-C12-alkylphenols which have been alkoxylated with ethylene oxide and/or propylene oxide are useful. Commercially available products of this kind include for example octylphenols or nonylphenols which have each been reacted with from 4 to 20 mol of ethylene oxide per mole of phenol. Other nonionic surfactants include ethoxylated C10-C24 fatty alcohols or ethoxylated C10-C24 fatty acids and also ethoxylated C10-C24 fatty amines or ethoxylated C10-C24 fatty amides. It is also possible to use polyhydric C3-C6-alcohols which have been partly esterified with C10-C24 fatty acids. These esters may additionally have been reacted with from 2 to 20 mol of ethylene oxide. Useful fatty alcohols for alkoxylation to prepare surfactants include for example palmityl alcohol, stearyl alcohol, myristyl alcohol, lauryl alcohol, oxo alcohols and also unsaturated alcohols, such as oleyl alcohol. The fatty alcohols are ethoxylated or propoxylated or ethoxylated and propoxylated to such an extent that the reaction products are soluble in water. Generally 1 mol of the above-indicated fatty alcohols is reacted with from 2 to 20 mol of ethylene oxide and optionally up to 5 mol of propylene oxide in such a way that surfactants having an HLB value of more than 8 are obtained.
Useful C3-C6-alcohols for partial esterification with or without ethoxylation include for example glycerol, sorbitol, mannitol and pentaerythritol. These polyhydric alcohols are partialy esterified with C10-C24 fatty acids, for example oleic acid, stearic acid or palmitic acid. The esterification with the fatty acids is carried on at most to such a degree as to leave at least one OH group of the polyhydric alcohol unesterified. Useful esterification products include for example sorbitan monooleate, sorbitan tristearate, mannitol monooleate, glycerol monooleate and glycerol dioleate. The aforementioned fatty esters of polyhydric alcohols that still contain at least one free OH group may be additionally reacted with ethylene oxide, propylene oxide or mixtures of ethylene oxide and propylene oxide for modification. Per mole of fatty ester it is preferable to use from 2 to 20 mol of the alkylene oxides mentioned. The degree of ethoxylation, as will be known, has an effect on the HLB value of nonionic surfactants. By suitably selecting the alkoxylating agent and the amount of alkoxylating agent it is possible to prepare surfactants having HLB values in the range from 3 to 20 in a technically simple manner.
A further group of useful substances are homopolymers of ethylene oxide, block copolymers of ethylene oxide and alkylene oxides, preferably propylene oxide, and also polyfunctional block copolymers formed, for example, by sequential addition of propylene oxide and ethylene oxide onto diamines.
It is further possible to use alkylpolyglycosides as marketed for example by Henkel under the trade marks APG(copyright), Glucopan(copyright) and Plantaren(copyright).
Nonionic surfactants can be used either alone or else mixed with each or one another.
Useful anionic surfactants include C8-C24-alkylsulfonates, which are preferably used in the form of the alkali metal salts, C8-C24-alkyl sulfates, which are preferably used in the form of the alkali metal or trialkanolammonium salts, e.g., triethanolammonium laurylsulfate, sulfosuccinic diesters, for example the sodium salt of di(2-ethylhexyl)sulfosuccinate, sulfosuccinic monoesters, for example sodium lauryl sulfosuccinate or disodium fatty alcohol polyglycol ether sulfosuccinate, C8-C24-alkylarylsulfonic acids and also the sulfuric monoesters of addition products of ethylene oxide with alkylphenols or fatty alcohols.
Examples of useful cationic surfactants are the salts of fatty amines, for example cocoammonium acetate, quaternary fatty acid amino esters, for example difatty acid isopropyl ester dimethylammonium methosulfate, quaternary fatty acid aminoamides, for example N-undecylenic propylamido N-trimethylammonium methosulfate, addition products of alkylene oxides with fatty amines or salts of fatty amines, for example pentaethoxystearylammonium acetate or ethoxylated methyloleinamine methosulfate and also long-chain alkylbenzyldimethylammonium compounds, such as C10-C22-alkylbenzyldimethylammonium chloride.
Examples of useful amphoteric surfactants are compounds bearing in one and the same molecule at least one quaternary ammonium cation and at least one carboxylate or sulfate anion, for example dimethylcarboxymethyl fatty acid alkylamidoammonium betaines or 3-(3-fatty acid amidopropyl)dimethylammonium 2-hydroxypropanesulfonates.
Ionic surfactants can be used alone or else mixed with each or one another.
Surfactants are used in amounts of from 0.001 to 5%, preferably from 0.01 to 2%, by weight based on the solids content of the polymer gel to be dried. Preference is given to the use of nonionic or anionic surfactants, and particular preference to the use of nonionic surfactants, such as the products of reacting 2-20 mol of ethylene oxide with the partial (C10-C24) fatty acid esters of polyhydric (C3-C6)-alcohols or the aforementioned esterification products which have not been reacted with ethylene oxide.
Useful release agents further include silicones such as polysiloxanes containing one or more selected from the group consisting of methyl, ethyl, propyl and phenyl as organic radicals. Preference is given to polydimethylsiloxanes and polymethylphenylsiloxanes and particular preference is given to polydimethylsiloxanes. Polysiloxanes may be chain or cyclical polymers, preference being given to those having a linear construction, especially polydimethylsiloxanes having a linear construction. It is further preferable to use silicones or polysiloxanes in the form of the commercially available products, which customarily constitute a mixture of substances and may also be modified silicones, for example aminosiloxanes. Preferred commercially available liquid silicones are the products generally referred to as silicone oils, and particular preference is given in turn to silicon oils based on dimethylpolysiloxane, specifically on polydimethylsiloxane having a linear construction. Preference is finally given to siloxanes having a 25xc2x0 C. viscosity of from 5 to 20,000 cSt, particularly preferably from 50 to 350 cSt, most preferably from 80 to 120 cSt, specifically those having a 25xc2x0 C. viscosity of about 100 cSt.
Examples of other, similarly useful release agentsare hexadecanol, octadecanol, hexadecyl acetate, octadecyl acetate, C12-C24-fatty acids and salts thereof, e.g., palmitic acid and its salts or stearic acid and its salts, methyl palmitate, butyl stearate, butyl oleate, hexylene glycol, octamethylene glycol, octadecane, eicosane, commercially available paraffin oils and paraffins wherein for example paraffinic, naphthenic and aromatic hydrocarbons may be included, having a melting point of not more than 100xc2x0 C. and a vapor pressure of not more than 0.1 mbar at 20xc2x0 C.
A further group of usefu-lrelease agents are polyglycols and polyglycol derivatives, especially polyalkylene glycols and polyalkylene glycol ethers, especially the mono- and dialkyl ethers. Particular preference is given to polyethylene glycols, polypropylene glycols, ethylene oxide-propylene oxide interpolymers, especially block polymers, mono- and di(C1-C4)alkyl, especially methyl, ethers of polyethylene glycol and polypropylene glycol, but also polyglycol ethers of higher molecular weight fatty alcohols. It is again preferable to use polyglycols and polyglycol ethers in the form of commercially available products, which customarily constitute a mixture of different substances, especially substances having different molecular weights.
In a preferred embodiment of the process, the release agent used is a neutralizing agent. Any neutralizing agent may be used which is also suitable for neutralizing the acidic hydrogel in the mincer. The neutralization in the mincer is preferably carried on to a degree of neutralization of not less than 50% by weight, preferably not less than 55% by weight, particularly preferably not less than 60% by weight. By additional treatment, for example by spraying hydrogel particles with the neutralizing agent or its aqueous solution, i.e., without mechanical shearing stress on the gel particles, the degree of neutralization is raised to the desired ultimate degree of neutralization. The neutralizing agent in the second step may be identical to or different from the neutralizing agent in the first step. The second neutralizing step is preferably carried out using aqueous sodium hydroxide solution.
The hydrogels which have thus been neutralized according to the invention and which have optionally been mixed with additional reactants and/or superabsorbent fines and which have subsequently been sprayed with a release agent in the manner described have an economically very advantageous drying rate for belt drying. In hot air drying at 180xc2x0 C. and an air velocity of 2 m/s they provide a standardized evaporation rate of not less than 90 kg/m2h, preferably not less than 120 kg/m2h, particularly preferably not less than 140 kg/m2h.
The hydrogel particles are subsequently dried. Belt drying is particularly preferable from an economic viewpoint. As well as factors to be optimized, such as the distribution of the hydrogel particles on the belt, the bed height of the hydrogel particles, the drying temperature or drying temperature profile, the relative humidity of the dryer air, air velocity, air distribution and air direction, it is the structure of the hydrogel particle bed which has a decisive influence on the rate of drying. The highest rates of drying are provided by loose, fluffy, separated gel particles, as provided by the process of the invention.
For the subsequent grinding of the dried hydrogel particles it is advantageous to cool the dried material to temperatures  less than 70xc2x0 C., preferably  less than 60xc2x0 C., particularly preferably  less than 50xc2x0 C., in the last section of the belt drying stage. The dried, cooled hydrogel particles are initially prebroken, for example by means of a knuckle-type crusher (precomminutor). The thus precomminuted hydrogel particles are then ground, preferably by means of one or more successive roll mills in order that the production of fines may be minimized. In a particularly preferred embodiment, the grinding is carried out twice, first via a coarse roll mill and then via a fine roll mill, and the latter may in turn be carried out in one or two stages. Sieving is carried out subsequently to set the particle size distribution, which is generally in the range from 100 to 1000 xcexcm, preferably from 120 to 850 xcexcm. Oversize particles may be resubmitted to grinding, while undersize particles may be recycled back into the production process, for example by mixing with the gel to be neutralized in the postneutralization step in the mincer, or be used for distinct purposes.
In a preerred embodiment of the invention, the absorption properties of the hydrophilic, highly swellable hydrogels thus obtained are still further improved by a subsequent, preferably covalent, surface postcrosslinking step. In this step, compounds capable of reacting with the carboxyl groups of the hydrogel by crosslinking are applied to the surface of the hydrogel particles, preferably in the form of an aqueous solution. Useful postcrosslinking agents include for example di- or polyglycidyl compounds such as phosphonyl diglycidyl ether or ethylene glycol diglycidyl ether, alkoxysilyl compounds, polyaziridines, polyamines or polyamidoamines and also their rection products with epichlorohydrin, polyols such as ethylene glycol, 1,2-propanediol, 1,4-butanediol, glycerol, di- and polyglycerol, pentaerythritol, sorbitol, the ethoxylates of these polyols and their esters with carboxylic acids or carbonic acid, ethylene carbonate, propylene carbonate, oxazolidone, bisoxazoline, polyoxazolines, di- and polyisocyanates. If necessary, acidic catalysts such as, for example, p-toluenesulfonic acid, phosphoric acid, boric acid or ammonium dihydrogenphosphate can be added.
Suitable mixing assemblies for spraying the hydrogel particles with crosslinker solution include for example Patterson-Kelly mixers, DRAIS turbulence mixers, Lxc3x6dige mixers, screw mixers, plate mixers, fluidized bed mixers, Schugi mixers. The spraying of the crosslinker solution may be followed by a temperature treatment step, preferably in a downstream dryer, at from 80 to 230xc2x0 C., preferably 80-190xc2x0 C., particularly preferably from 100 to 160xc2x0 C., for from 5 minutes to 6 hours, preferably from 10 minutes to 2 hours, particularly preferably form 10 minutes to 1 hour; lysis products may be removed as well as solvent fractions.
In a particularly preferred embodiment of the invention the hydrophilicity of the hydrogel particle surface is additionally modified through formation of metal complexes. The formation of metal complexes on the outer shell of the hydrogel particles is effected by spraying with solutions of divalent or more highly valent metal salt solutions to allow the metal cations to react with the carboxyl groups of hydrogel to form complexes. Examples of di- or more highly valent metal cations are Mg2+, Ca2+, Al3+, Sc3+, Ti4+, Mn2+, Fe2+/Fe3+, Co2+, Ni2+, Cu+/Cu2+, Zn2+, Y3+, Zr4+, Ag+, La3+, Ce4+, Hf4+, and Au+/Au3+, preferred metal cations being Mg2+, Ca2+, Al3+, Ti4+, Zr4+ and La3+; particularly preferred metal cations are Al3+, Ti4+ and zr4+. Metal cations may be used alone or else as a mixture with each or one another. Of the metal cations mentioned, any metal salt possessing sufficient solubility in the solvent to be used is suitable. Metal salts with weakly complexing anions, for example, chloride, nitrate or sulfate, are particularly suitable. Useful solvents for the metal salts include water, alcohols, DMF, DMSO and mixtures thereof. Particular preference is given to water and water/alcohol mixtures, for example water/methanol or water/1,2-propanediol.
The spraying of the metal salt solution onto the hydrogel particles may take place both before and after the surface postcrosslinking of hydrogel particles. In a particularly preferred process, the spraying on of the metal salt solution takes place in the same step as the spraying on of the crosslinker solution, the two solutions being sprayed on separately in succession or simultaneously via two nozzles, or crosslinker solution and metal salt solution may be sprayed on together via a single nozzle.
Optionally, the hydrogel particles may be further modified by admixture of finely divided inorganic solids, for example silica, alumina, titania and iron(II) oxide to further augment the effects of the surface aftertreatment. Particular preference is given to the admixture of hydrophilic silica or of alumina having an average primary particle size of from 4 to 50 nm and a specific surface area of 50-450 m2/g. The admixture of finely divided inorganic solids preferably takes place after the surface modification through crosslinking/complexing, but may also be carried out before or during these surface modifications.
Hydrogels of the invention are notable for outstanding absorbency coupled with high gel strength and low levels of extractabies and are therefore very useful as absorbants for water and aqueous fluids, especially body fluids, for example urine or blood, for example in hygiene articles such as, for example, infant and adult diapers, sanitary napkins, tampons and the like. But they may also be used as soil improvers in agriculture and market gardening, as moisture binders in cable sheathing and also for thickening aqueous wastes.
Description of test methods used in examples:
CRC (Centrifuge Retention Capacity):
0.2 g of hydrogel (particle size fraction 106-850 xcexcm) is weighed into a teabag 60xc3x9760 mm in size, which is subsequently welded shut. The teabag is then placed in an excess of 0.9% by weight sodium chloride solution (at least 1.25 1 of sodium chloride solution/1 g of hydrogel). After a swelling time of 20 minutes, the teabag is removed from the sodium chloride solution and centrifuged at 250 g for three minutes. The centrifuged teabag is weighed to determine the amount of liquid retained by the hydrogel.
Extractables (16 h):
1 g of hydrogel (particle size fraction 106-850 xcexcm) is stirred into 200 ml of 0.9% by weight sodium chloride solution. The beaker is sealed and the mixture is stirred for 16 h. This is followed by filtration through a 0.22 xcexcm filter and determination of the level of extractables by an acid-base titration of the carboxyl groups (titration with 0.1 normal NaOH to pH 10, then with 0.1 normal HCl to pH 2.7).
AUL (Absorbency under Load):
Absorbency under Load (AUL) was determined in known manner as described for example in EP-A-0 339 461. AUL 70 indicates a measurement of the absorbency under a load of 70 g/cm2, the area coverage of the hydrogel particles (partice size fraction 106-850 xcexcm) in the measuring cell being 0.032 g/cm2.
Gel Column Test:
The apparatus is a glass column 2.6 cm in diameter and not less than 40 cm in length which has a 0 porosity frit and a tap at the lower end. The glass column drains into a beaker standing on a balance. Its weight is recorded continuously, for example by means of a computer. To carry out the gel column test, 1 g of hydrogel is allowed to swell in 100 g of 0.9% by weight NaCl solution for 5 minutes. The swollen gel is transferred into the glass column. To condition the gel, 100 ml of 0.9% by weight NaCl solution are added, the tap is opened and the emergence of liquid having passed through the swollen gel layer is awaited. The tap is then closed and another 100 g of 0.9% by weight NaCl solution are added. After opening of the tap, the amount of liquid passing through is recorded as a function of time. The amount of liquid which has passed through after 60 seconds is the 60 s flowthrough value.
Gel Strength:
Gel strength is measured using a Carri-Med-Stress rheometer having a plate-plate configuration. 1 g of hydrogel is allowed to swell in 60 g of 0.9% by weight sodium chloride solution for 24 hours and subsequently the storage modulus Gxe2x80x2 of this swollen gel is measured as a function of the shear stress at a frequency of 1 Hz. The plateau value is reported as the gel strength.
Determination of Standardized Evaporation Rate:
The standardized evaporation rate is determined using a convection belt dryer simulator under the following standardized conditions: